This invention relates to the production of titaninum diboride and is directed to an improved process for so doing.
A variety of methods have been developed for producing metal borides such as titanium diboride, as follows:
1. Direct combination of the metal with boron or a metal hydride and boron.
2. Hydrogen reaction of a boron halide with a metal source such as an oxide.
3. Electrolysis of a melt consisting of a metal oxide, boron oxide, and an alkali flux.
4. Reduction of a metal oxide with boron (borothermic) or co-reduction with oxide by carbon (carbothermic).
5. Reduction of oxides by metals.
6. Arc-plasma reaction from vapor reactants.
Methods 1-3 are used mainly in laboratory experimentation. Method 4 has been employed commercially but the borothermic method requires large amounts of boron over what the final product contains and is thus too wasteful for high production levels. The carbothermic method, however, is the one generally used to produce titanium diboride. In this method, the raw materials are heated to a temperature in the range of 1800.degree.-2000 .degree.C. until titanium diboride is formed. The powdered titanium diboride ultimately recovered requires forming temperatures in about this same range. Method 5 has been attempted but found to produce contaminants of other borides as well as borides and titanates, and has not been considered a commercially viable method. Method 6 produces pyrophoric titanium diboride from expensive reagents.
The principle disadvantages of the above methods are, for 1, expensive raw materials and the likelihood of formation of potentially explosive boranes; for 2-4, expensive raw materials and reaction vessels; for 5, production of contaminant borides, borates and titanates; and for 6, expensive raw materials, reaction vessels and formation of pyrophoric titanium diboride. The carbothermic method, method 4, is currently being developed commercially to produce sub-micron, crystalline titanium diboride at reduced cost. Based upon a raw material cost of about $14.00 per pound, titanium diboride costing about $36.00-40.00 per pound is possible at the present time. High temperatures are required in the processing; and in order to obtain the product in sub-micron size, final grinding, a source of contamination, is also required. The resultant powdered, sub-micron titanium diboride is crystalline (hexagonal) and is difficult to sinter. The powder must be vacuum sintered at 2200.degree. C. or hot pressed at 1800.degree.-2200.degree. C., and even at carefully controlled processing conditions, micro-stresses are caused by thermal expansion of individual crystallites by different amounts in the a and c directions.
In accord with this invention, a highly reactive, sub-micron titanium diboride powder of variable crystallinity (amorphous to crystalline) and purity, dependent on control of processing parameters, is produced in situ from an exothermic reaction mixture, thereby eliminating the need for grinding, and attaining, for the first time it is believed, sub-micron titanium diboride powder of highly reactive form. By "highly reactive" is meant sufficient reactivity to significantly decrease the temperature required for sintering. Products of this invention may be sintered by hot pressing at temperatures in the range of 1300.degree.-1500.degree. C. as opposed to the normally required range of 1800.degree.-2200.degree. C.
A process parameter which affects whether, or the degree to which the resultant product is amorphous, is the rapidity with which the composition is cooled after the exothermic reaction and the ability to suspend the powder during the reaction preventing localized sintering and grain growth.
The purity of the product, e.g., the absence of borates and titanates therein is strongly dependent upon control of the weight amounts of the components of the reaction mixture. The absence of borates and titanates formed during the leaching step is strongly dependent upon the pH control of the leaching solution. It has been found that if the magnesium and the boron oxide are present in amounts of about 5-30% in excess of stoichiometric in the reaction mixture, the titanium diboride will be a mixture of amorphous and crystalline forms. When the magnesium and boron oxide are present in excess of stoichiometric, the best results are attained.
The process of this invention appears inherently to provide the titanium diboride in sub-micron, highly reactive form.
The process of the invention is practiced successfully when the reducing metal of the exothermic reaction is magnesium.
These and other objects of the invention will become apparent as this description proceeds.